FOWT platform motion is strange at large mooring line lengths

Hello community,

I’ve been examining the effect of increasing the length of mooring lines in FASTv8 on the motion of a three-cylinder semi-submersible FOWT platform. I’ve noticed that beyond a certain cable length, the platform surge ceases to increase (see the attached image).

Has anyone encountered this issue? It seems to me as though there may be some function within FASTv8 or MoorDyn that places limits on the mooring line length.

(i) I’ve only included rigid-body degrees of freedom.
(ii) There are no waves (i.e. the still water option is selected).
(iii) The yaw and blade pitch angles are maintained at zero degrees.
(iv) The generator torque is controlled using a common function based on the optimal TSR and maximum power coefficient.
(v) The wind is steady and uniform at 11.4 m/s.

Dear Ali,

I don’t believe that MoorDyn places a limit on the mooring line length.

I would likely have to know about your simulation set up to pinpoint exactly the cause of the behavior you are seeing, but I would guess that the reason for the limit on surge motion has something to do with response parameters you are not plotting e.g. rotor speed, platform pitch, etc.

Best regards,

Thank you for the reply Dr. Jonkman,

Checking the other FOWT states was my first thought as well, but I did not notice anything unusual (the turbine is definitely not capsizing!).
For your reference, I simply took the unaltered NREL Baseline 5MW OC4DeepCwindSemi certification files and made the following changes:

(i) Disabled all DOF’s except for those corresponding to platform motion and generator rotation (ElastoDyn)
(ii) Set all initial conditions to zero, except for the rotor speed IC; which was set to 11.94 RPM (ElastoDyn)
(iii) Set WaveMod to zero in order to model still water (HydroDyn)
(iv) Switched off blade pitch, yaw, and generator brake control, and implemented a simple VS generator controller (ServoDyn):
— VS_RtGnSp = 1158.18 RMP
— VS_RtTq = 43340 N-m
— VS_Rgn2K = 0.0323093484139365 N-m/(RPM^2)
— VS_SlPc = 2
(v) Removed turbulence and defined a steady wind speed of 11.4 m/s (InflowWind)

As an example, here are the resulting steady values for the states (after 1200 s) corresponding to cable lengths of 1000 and 950 m:

Cable length: --------- 1000 m ----------- 950 m
Rotor speed: --------- 11.92 RPM ------ 11.91 RPM
Platform surge: ----- 62.44 m --------- 58.89 m
Platform sway: ----- -1.249 m -------- -0.5424 m
Platform heave: ----- 0.3162 m -------- 0.2969 m
Platform roll: -------- 0.3089 deg ------ 0.2660 deg
Platform pitch: ------ 1.691 deg ------- 1.807 deg
Platform yaw: ------- -1.919 deg ------ -0.7904 deg

I don’t notice anything unusual about the remaining states. On the other hand, an additional 50 m of cable length only increases the platform surge by 3.55 m…

For now, I’ve implemented my own version of MoorDyn based on Matthew Hall’s paper and this issue has been resolved (the platform surge continues to grow almost linearly with cable length).
I will spend some more time trying to find my mistake in FAST.

Thanks again for your time and your suggestions Dr. Jonkman,