Moordyn convergence problem consultation

Wind & Water Computer-Aided Engineering Software Tools
1

Dear everyone,
I want to add a synthetic fiber to the floating wind turbine mooring system to replace the iron chain. I use Moordyn as the mooring module, but I have encountered a problem where my Moordyn always fails to initialize, as shown in the following picture

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On a mooring line, I am divided into three sections. The first section closest to the platform is the iron chain, the second section is the synthetic fiber, and the third section is the iron chain. I can determine the length of these three sections, but I cannot determine the specific node positions. After several failed experiences, I believe that the failure of catenary linearization was caused by incorrect coordinate settings at POINT. Do you have any good methods to determine the coordinate positions that can help me simulate my idea smoothly?

--------------------- MoorDyn Input File ------------------------------------
MoorDyn v2 sample input file
---------------------- LINE TYPES -----------------------------------
TypeName   Diam     Mass/m     EA            BA/-zeta    EI         Cd     Ca     CdAx   CaAx
(name)     (m)      (kg/m)     (N)           (N-s/-)     (N-m^2)    (-)    (-)    (-)    (-)
Chain       0.333    685.0   3.27E9         -1           0          1.11   0.82   0.2    0.27     !chain
Polyester   0.205    45.10   3.97E8         -1           0          1.2    0.82   0.2    0.27     !polyester static
Polyester1  0.205    45.10   1.01E8|1.01E8  -1|-1        0          1.2    0.82   0.2    0.27     !polyester line1 viscoelastic
Polyester23 0.205    45.10   1.01E8|1.91E8  -1|-1        0          1.2    0.82   0.2    0.27     !polyester line2/3 viscoelastic
---------------------- POINTS ---------------------------------------
ID   Attachment  X       Y     Z          Mass   Volume  CdA    Ca
(#)   (word/ID) (m)     (m)   (m)         (kg)   (m^3)   (m^2)  (-)
1   Vessel  -58.000    0.000   -14.000     0    0    0    0
2   Free    -71.522    0.000   -37.550     0    0    0    0
3   Free    -81.632    0.000   -51.230     0    0    0    0
4   Fixed   -837.600   0.000   -200.000    0    0    0    0
5   Vessel   29.000    50.229  -14.000     0    0    0    0
6   Free     37.146    62.761  -34.387     0    0    0    0
7   Free     42.170    71.488  -48.09      0    0    0    0
8   Fixed    418.800   725.383 -200.000    0    0    0    0
9   Vessel   29.000   -50.229  -14.000     0    0    0    0
10  Free     37.146   -62.761  -34.387     0    0    0    0
11  Free     42.170   -71.488  -48.09      0    0    0    0
12  Fixed    418.800  -725.383 -200.000    0    0    0    0
---------------------- LINES ----------------------------------------
ID   LineType   AttachA  AttachB  UnstrLen  NumSegs  LineOutputs
(#)   (name)     (ID)     (ID)      (m)       (-)      (-)
1     Chain       2         1      30.00      2        -
2     Polyester  3         2      20.00      10       ts
3     Chain       4         3     800.00      80       -
4     Chain       6         5      30.00      2        -
5     Polyester  7         6      20.00      10       t
6     Chain       8         7     800.00      80       -
7     Chain       10        9      30.00      2        -
8     Polyester  11        10     20.00      10       -
9     Chain       12        11    800.00      80       -
---------------------- OPTIONS -----------------------------------------
0.001         dtM           time step to use in mooring integration (s)
3e6           kbot          bottom stiffness (Pa/m)
3e5           cbot          bottom damping (Pa-s/m)
0.01           dtIC          time interval for analyzing convergence during IC gen (s)
60            TmaxIC        max time for ic gen (s)
0.01          threshIC      threshold for IC convergence (-)
0             MU_KT          axial line friction coefficients
0             MU_KA          transverse line friction coefficients
"wavedynamic.dat"             WaterKin              the wave elevations are provided in a grid (-)
---------------------- OUTPUTS -----------------------------------------
FAIRTEN1
FAIRTEN2
FAIRTEN3
ANCHTEN1
ANCHTEN2
ANCHTEN3
END
------------------------- need this line -------------------------------------

Best regards,

2

Hi @Yushun.Fu,

Your point positions don’t necessarily look bad here. The MoorDyn initialization routine allows for points to be placed at a “best guess” position, and then lets the system “fall” into its steady state before beginning the simulation. A few things can be causing your issue:

  1. If your platform/vessel is not located at 0,0,0 then there can be large initial transients because the points labeled vessel will have their locations overwritten by that position (as if they have been defined in the vessel reference frame).
  2. Your dtm value seems large. Decreasing this will improve numerical stability
  3. Similarly, you have 10 segments for a 20m polyester rope, but only 2 segments for a 30m chain. Increasing the number of segments will also improve stability. I would suggest ~2m long segments for your short lines. 80 should be sufficient for the 800m chains.
  4. You should add the CdScaleIC flag to the options list and set it equal to 2 or 3. It defaults at 4, which can cause instabilities. You can read more about initialization drag scaling in the docs. Note that MoorDyn-F is what is used in OpenFAST.
3

Thank you for your reply. I have successfully resolved this issue！