MoorDyn one segment vs. three segments

Wind & Water Computer-Aided Engineering Software Tools
1

Dear all,

I am simulating a semisubmersible floating wind turbine for irregular wave+wind+current. There are three segments for the mooring lines. I simulated two cases: one segment mooring line and three segments mooring line, and found that mooring line tension, wind turbine thrust are comparable for the two cases, but there is a large difference for platform surge.
My concern is the coordinates of the line connections. How to specify these coordinates. At first when I randomly set the values, I got some warnings: “catenary solver failed for one or more lines, using linear node spacing”. After some adjustment, the warnings were eliminated. But still, the coordinates are based on assumptions.

I have following queries, any comments and suggestions will be greatly appreciated.

  1. Is there a method to locate the connection coordinates?
  2. How important is the accuracy of the connection location, can the mooring line be modelled by specifying the unstretched segment length instead of connection coordinates in MoorDyn?
  3. For the one line simulation, weighted average per length was used for mooring line properties, is it correct?

Results for one mooring line (average from 3600-7200 secs):

FAIRTEN1 FAIRTEN2 FAIRTEN3 ANCHTEN1 ANCHTEN2 ANCHTEN3
(N) (N) (N) (N) (N) (N)
1.58E+06 4.07E+06 4.17E+06 8.13E+05 3.27E+06 3.37E+06
PtfmSurge PtfmSway PtfmHeave PtfmRoll PtfmPitch PtfmYaw
(m) (m) (m) (deg) (deg) (deg)
15.308 0.440 -0.109 0.343 4.657 0.572
RotTorq RotThrust
(kN-m) (kN)
9685.297913 1727.854552

Results for three segments mooring line:

FAIRTEN7 FAIRTEN8 FAIRTEN9 ANCHTEN1 ANCHTEN2 ANCHTEN3
(N) (N) (N) (N) (N) (N)
1.52E+06 4.19E+06 4.29E+06 8.89E+05 3.39E+06 3.49E+06
PtfmSurge PtfmSway PtfmHeave PtfmRoll PtfmPitch PtfmYaw
(m) (m) (m) (deg) (deg) (deg)
11.076 0.436 -0.099 0.353 4.418 0.563
RotTorq RotThrust
(kN-m) (kN)
9695.272214 1723.659372

Best regards,
Xiaoqin

2

After checking the documents, the connection coordinates should not affect the final result, they are just initial guess.

Then, the difference between the surge is caused by conversion of the properties of three segments into one line. Three segments should give more accurate result as the mooring line was designed to have three segments. But for now, compared to model test, three segments simulation gives 22% underestimation for surge, while one line simulation gives 8% overestimation.

What’s the effect of evening out the difference between segments? Any comments are greatly appreciated.

Regards,
Xiaoqin

3

Hi Xiaoqin,

I’m glad you figured out some of the questions already. I agree with your conclusions.

In general, the effect of weight on a mooring line depends strongly on the location of that weight along the line, so it is difficult to replace a three-section mooring line with an equivalent one-section mooring line. The tension and stiffness could be tuned for a given position, but it could be quite difficult to achieve the same force-displacement relationship over the full range of possible offsets.

If you wanted to explore this further, you could use a fast quasi-static model like MoorPy (GitHub - NREL/MoorPy) to quickly try out different designs.

Best,
Matt