Regarding the Issue of Modifying Drag Coefficients Based on Depth in HydroDyn

Wind & Water Computer-Aided Engineering Software Tools
1

Dear Jason,

I found your paper’s conclusion( Redirecting ) on the low-frequency surge response of the OC6 floating wind turbine very insightful—specifically, how increasing the drag coefficient near the still water level improves prediction of low-frequency forces while preserving surge decay and forced motion behavior.

Inspired by this, I applied a similar adjustment to the OC4 model. I changed the HydroDyn Upper Column MCoefMod from 3 to 2 and scaled the drag coefficient from 0.61 to 2.44. Simulations were run under LC 5.3 conditions for 4000 seconds, with other settings kept mostly default.

However, the modified model produced a lower low-frequency surge PSD compared to the original. Could this be due to differences between OC6 and OC4, or possibly the influence of the linear damping coefficients mentioned in your paper?

I would appreciate your guidance on this discrepancy.


-------- SIMPLE CYLINDRICAL-MEMBER HYDRODYNAMIC COEFFICIENTS (model 1) ---------
     SimplCd    SimplCdMG    SimplCa    SimplCaMG    SimplCp    SimplCpMG   SimplAxCd  SimplAxCdMG   SimplAxCa  SimplAxCaMG  SimplAxCp   SimplAxCpMG    SimplCb    SimplCbMG
       (-)         (-)         (-)         (-)         (-)         (-)         (-)         (-)         (-)         (-)         (-)         (-)            (-)         (-)
       0.00        0.00        0.00        0.00        1.00        1.00        0.00        0.00        0.00        0.00        1.00        1.00           1.00        1.00
-------- SIMPLE RECTANGULAR-MEMBER HYDRODYNAMIC COEFFICIENTS (model 1) ---------
     SimplCdA    SimplCdAMG    SimplCdB    SimplCdBMG    SimplCaA    SimplCaAMG    SimplCaB    SimplCaBMG    SimplCp    SimplCpMG   SimplAxCd  SimplAxCdMG   SimplAxCa  SimplAxCaMG  SimplAxCp   SimplAxCpMG  SimplCb  SimplCbMG
       (-)         (-)           (-)         (-)           (-)         (-)           (-)         (-)           (-)         (-)         (-)         (-)          (-)         (-)         (-)         (-)          (-)       (-)
       0.0         0.0           0.0         0.0           0.0         0.0           0.0         0.0           0.0         0.0         0.0         0.0          0.0         0.0         0.0         0.0          1.0       1.0
------ DEPTH-BASED CYLINDRICAL-MEMBER HYDRODYNAMIC COEFFICIENTS (model 2) -------
             4   NCoefDpthCyl    - Number of depth-dependent cylindrical member coefficients (-)
Dpth      DpthCd   DpthCdMG   DpthCa   DpthCaMG       DpthCp   DpthCpMG   DpthAxCd   DpthAxCdMG   DpthAxCa   DpthAxCaMG   DpthAxCp   DpthAxCpMG   DpthCb   DpthCbMG
(m)       (-)      (-)        (-)      (-)            (-)      (-)        (-)        (-)          (-)        (-)          (-)        (-)           (-)      (-)
15 2.44 0 0 0 0 0 0 0 0 0 0 0 0 0 
-3 2.44 0 0 0 0 0 0 0 0 0 0 0 0 0
-4 0.61 0 0 0 0 0 0 0 0 0 0 0 0 0
-15 0.61 0 0 0 0 0 0 0 0 0 0 0 0 0
------ DEPTH-BASED RECTANGULAR-MEMBER HYDRODYNAMIC COEFFICIENTS (model 2) -------
             0   NCoefDpthRec    - Number of depth-dependent rectangular member coefficients (-)
Dpth    DpthCdA   DpthCdAMG    DpthCdB   DpthCdBMG   DpthCaA   DpthCaAMG   DpthCaB   DpthCaBMG     DpthCp   DpthCpMG   DpthAxCd   DpthAxCdMG   DpthAxCa   DpthAxCaMG   DpthAxCp   DpthAxCpMG   DpthCb   DpthCbMG
(m)       (-)       (-)          (-)       (-)         (-)       (-)        (-)        (-)          (-)        (-)       (-)        (-)          (-)        (-)          (-)        (-)          (-)      (-)
------ MEMBER-BASED CYLINDRICAL-MEMBER HYDRODYNAMIC COEFFICIENTS (model 3) ------
            25   NCoefMembersCyl - Number of member-based cylindrical member coefficients (-)
MemberID    MemberCd1     MemberCd2    MemberCdMG1   MemberCdMG2    MemberCa1     MemberCa2    MemberCaMG1   MemberCaMG2    MemberCp1     MemberCp2    MemberCpMG1   MemberCpMG2   MemberAxCd1   MemberAxCd2  MemberAxCdMG1 MemberAxCdMG2  MemberAxCa1   MemberAxCa2  MemberAxCaMG1 MemberAxCaMG2  MemberAxCp1  MemberAxCp2   MemberAxCpMG1   MemberAxCpMG2    MemberCb1     MemberCb2    MemberCbMG1   MemberCbMG2
   (-)         (-)           (-)           (-)           (-)           (-)           (-)           (-)           (-)           (-)           (-)           (-)           (-)           (-)           (-)           (-)           (-)           (-)           (-)           (-)           (-)           (-)           (-)           (-)           (-)              (-)           (-)           (-)           (-)
    1          0.56          0.56          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00             1.00          1.00          1.00          1.00           ! Main Column
    2          0.61          0.61          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00             1.00          1.00          1.00          1.00           ! Upper Column 1
    3          0.61          0.61          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00             1.00          1.00          1.00          1.00           ! Upper Column 2
    4          0.61          0.61          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00             1.00          1.00          1.00          1.00           ! Upper Column 3
    5          0.68          0.68          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00             1.00          1.00          1.00          1.00           ! Base Column 1
    6          0.68          0.68          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00             1.00          1.00          1.00          1.00           ! Base Column 2
    7          0.68          0.68          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00             1.00          1.00          1.00          1.00           ! Base Column 3
   23          0.68          0.68          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00             1.00          1.00          1.00          1.00           ! Base column cap 1
   24          0.68          0.68          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00             1.00          1.00          1.00          1.00           ! Base column cap 2
   25          0.68          0.68          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00             1.00          1.00          1.00          1.00           ! Base column cap 3
    8          0.63          0.63          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00             1.00          1.00          1.00          1.00           ! Delta Pontoon, Upper 1
    9          0.63          0.63          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00             1.00          1.00          1.00          1.00           ! Delta Pontoon, Upper 2
   10          0.63          0.63          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00             1.00          1.00          1.00          1.00           ! Delta Pontoon, Upper 3
   11          0.63          0.63          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00             1.00          1.00          1.00          1.00           ! Delta Pontoon, Lower 1
   12          0.63          0.63          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00             1.00          1.00          1.00          1.00           ! Delta Pontoon, Lower 2
   13          0.63          0.63          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00             1.00          1.00          1.00          1.00           ! Delta Pontoon, Lower 3
   14          0.63          0.63          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00             1.00          1.00          1.00          1.00           ! Y Pontoon, Upper 1
   15          0.63          0.63          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00             1.00          1.00          1.00          1.00           ! Y Pontoon, Upper 2
   16          0.63          0.63          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00             1.00          1.00          1.00          1.00           ! Y Pontoon, Upper 3
   17          0.63          0.63          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00             1.00          1.00          1.00          1.00           ! Y Pontoon, Lower 1
   18          0.63          0.63          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00             1.00          1.00          1.00          1.00           ! Y Pontoon, Lower 2
   19          0.63          0.63          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00             1.00          1.00          1.00          1.00           ! Y Pontoon, Lower 3
   20          0.63          0.63          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00             1.00          1.00          1.00          1.00           ! Cross Brace 1
   21          0.63          0.63          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00             1.00          1.00          1.00          1.00           ! Cross Brace 2
   22          0.63          0.63          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00          0.00             1.00          1.00          1.00          1.00           ! Cross Brace 3
------ MEMBER-BASED RECTANGULAR-MEMBER HYDRODYNAMIC COEFFICIENTS (model 3) ------
            0   NCoefMembersRec - Number of member-based rectangular member coefficients (-)
MemberID    MemberCdA1     MemberCdA2    MemberCdAMG1   MemberCdAMG2    MemberCdB1     MemberCdB2    MemberCdBMG1   MemberCdBMG2    MemberCaA1     MemberCaA2    MemberCaAMG1   MemberCaAMG2    MemberCaB1     MemberCaB2    MemberCaBMG1   MemberCaBMG2    MemberCp1     MemberCp2    MemberCpMG1   MemberCpMG2   MemberAxCd1   MemberAxCd2  MemberAxCdMG1 MemberAxCdMG2  MemberAxCa1   MemberAxCa2  MemberAxCaMG1 MemberAxCaMG2  MemberAxCp1  MemberAxCp2   MemberAxCpMG1   MemberAxCpMG2   MemberCb1     MemberCb2    MemberCbMG1   MemberCbMG2
   (-)         (-)            (-)           (-)            (-)             (-)            (-)           (-)            (-)             (-)            (-)           (-)            (-)             (-)            (-)           (-)            (-)             (-)           (-)          (-)           (-)           (-)           (-)          (-)           (-)            (-)           (-)          (-)           (-)              (-)           (-)          (-)           (-)            (-)           (-)          (-)           (-)
-------------------- MEMBERS -------------------------------------------------
            25   NMembers       - Number of members (-)
MemberID  MJointID1  MJointID2  MPropSetID1  MPropSetID2  MSecGeom    MSpinOrient   MDivSize   MCoefMod  MHstLMod  PropPot   [MCoefMod=1: use simple coeff table, 2: use depth-based coeff table, 3: use member-based coeff table] [ PropPot/=0 if member is modeled with potential-flow theory]
  (-)        (-)        (-)         (-)          (-)      (switch)       (deg)        (m)      (switch)  (switch)  (flag)
   1          1          2           1            1           1            0         1.0000        3         1      TRUE           ! Main Column
   2          3          4           2            2           1            0         1.0000        2         1      TRUE           ! Upper Column 1
   3          5          6           2            2           1            0         1.0000        2         1      TRUE           ! Upper Column 2
   4          7          8           2            2           1            0         1.0000        2         1      TRUE           ! Upper Column 3
   5         42          3           3            3           1            0         1.0000        3         1      TRUE           ! Base Column 1
   6         43          5           3            3           1            0         1.0000        3         1      TRUE           ! Base Column 2
   7         44          7           3            3           1            0         1.0000        3         1      TRUE           ! Base Column 3
  23          9         42           3            3           1            0         1.0000        3         1      TRUE           ! Base column cap 1
  24         10         43           3            3           1            0         1.0000        3         1      TRUE           ! Base column cap 2
  25         11         44           3            3           1            0         1.0000        3         1      TRUE           ! Base column cap 3
   8         12         13           4            4           1            0         1.0000        3         1      TRUE           ! Delta Pontoon, Upper 1
   9         14         15           4            4           1            0         1.0000        3         1      TRUE           ! Delta Pontoon, Upper 2
  10         16         17           4            4           1            0         1.0000        3         1      TRUE           ! Delta Pontoon, Upper 3
  11         18         19           4            4           1            0         1.0000        3         1      TRUE           ! Delta Pontoon, Lower 1
  12         20         21           4            4           1            0         1.0000        3         1      TRUE           ! Delta Pontoon, Lower 2
  13         22         23           4            4           1            0         1.0000        3         1      TRUE           ! Delta Pontoon, Lower 3
  14         24         25           4            4           1            0         1.0000        3         1      TRUE           ! Y Pontoon, Upper 1
  15         26         27           4            4           1            0         1.0000        3         1      TRUE           ! Y Pontoon, Upper 2
  16         28         29           4            4           1            0         1.0000        3         1      TRUE           ! Y Pontoon, Upper 3
  17         30         31           4            4           1            0         1.0000        3         1      TRUE           ! Y Pontoon, Lower 1
  18         32         33           4            4           1            0         1.0000        3         1      TRUE           ! Y Pontoon, Lower 2
  19         34         35           4            4           1            0         1.0000        3         1      TRUE           ! Y Pontoon, Lower 3
  20         36         37           4            4           1            0         1.0000        3         1      TRUE           ! Cross Brace 1
  21         38         39           4            4           1            0         1.0000        3         1      TRUE           ! Cross Brace 2
  22         40         41           4            4           1            0         1.0000        3         1      TRUE           ! Cross Brace 3

1
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Best regards,

2

Dear @Chen.Yang,

Can you clarify what your starting point is? Are you starting with the OpenFAST model of the NREL 5-MW baseline turbine atop the OC4-DeepCwind semisubmersible available from the OpenFAST r-test: r-test/glue-codes/openfast/5MW_OC4Semi_WSt_WavesWN at main · OpenFAST/r-test · GitHub?

Best regards,

3

Dear Jason,

Thanks for your reply.

I checked the source code, and it’s from the GitHub r-test, which is completely consistent with the link you provided. I used the default settings of 5MW_OC4Semi_WSt_WavesWN to run the original case shown in the graph above. Then, I primarily modified the drag coefficient based on depth. The main modifications are as follows. Due to the forum’s word limit, I didn’t copy the entire code earlier, and the code I provided before was the modified version, which may have caused misunderstanding.

------ DEPTH-BASED CYLINDRICAL-MEMBER HYDRODYNAMIC COEFFICIENTS (model 2) -------
             4   NCoefDpthCyl    - Number of depth-dependent cylindrical member coefficients (-)
Dpth      DpthCd   DpthCdMG   DpthCa   DpthCaMG       DpthCp   DpthCpMG   DpthAxCd   DpthAxCdMG   DpthAxCa   DpthAxCaMG   DpthAxCp   DpthAxCpMG   DpthCb   DpthCbMG
(m)       (-)      (-)        (-)      (-)            (-)      (-)        (-)        (-)          (-)        (-)          (-)        (-)           (-)      (-)
15 2.44 0 0 0 0 0 0 0 0 0 0 0 0 0 
-3 2.44 0 0 0 0 0 0 0 0 0 0 0 0 0
-4 0.61 0 0 0 0 0 0 0 0 0 0 0 0 0
-15 0.61 0 0 0 0 0 0 0 0 0 0 0 0 0

Best regards.

4

Hi @Chen.Yang,

To capture the slow drift motion, you also need to use a suitable wave stretching model. We suggest using vertical wave stretching for this purpose. You can enable vertical wave stretching by setting WaveStModto 1 in the SeaState input file.

Best,