I am checking the mooring forces calculated by OpenFAST with usage of MoorDyn (a screeshot of input file attached). I found the minimum tension force at one of the fairlead is close to zero, which is not reasonable for me. The issue only happened when simulating the FOWT for extreme environmental conditions (for example when wind speed = 51m/s, hs = 12m, tp = 14s, current speed = 1.7m/s), while for the normal conditions the tension force seem correct. Could you please advice the reason that may cause this problem?
In the simulation where you are experiencing low mooring line tension, are you seeing other abnormalities in the response, such as excessive blade or tower deflection, or large floater motion? For this case, is the rotor parked or idling with the blades feathered; do you have a sizeable yaw error between the rotor and wind?
Dear Dr Jonkman,
Thanks for your reply.
The abnormal mooring (close to zero) forces are mostly observed when the platform has significantly surge&sway (though I don’t think the curves show any abnormality), which can be related to the change in rotor thrust (as illustraed in the last two figures). I don’t see any excessive blade or tower deflection during the simulation.
The rotor is parked and feathered to 90 degree. As you can see in the first figure attached, there is an angle between the rotor axis and the inflow wind.
The lowest tension force observed in this case is 10.85 kN, however the mass density of the line is 459.77 kg/m, does this mean the length of the free hanging part of the line is only 2 m?
Equating vertical line tension with the weight minus buoyancy of the line is true for a slack line in quasi-static conditions, but not in dynamic conditions.
The sizeable yaw error with blades feathered under idling conditions can lead to blade instabilities, which would result in sizeable system motion, as discussed in the following forum topic: Designing for yaw errors using FAST. But if this was the case in your model, I’d expect that you’d see sizeable blade deflection, which it doesn’t sound like you are seeing.
You haven’t shown the coordinate system, but I assume that the large 20-m mean surge and -10 m mean sway is motion toward the lower-right corner of your figure, in the opposite direction of the rotor thrust. Thus, from your figure, it appears that the surge and sway motion is driven more by current than by rotor thrust. Is that correct? Perhaps the issue is purely related to the strong current? If you’ve enabled second-order hydrodynamics, surge and sway motion could also be driven by mean-drift effects.
Dear Dr Jonkman,
Thanks for your reply, I think this is a proper explaination if no sizeable deflection/motion was observed in this simulation.
I agree that the mean offset of the platform is caused by the current, while the fluctuation should be the result of rotor thrust variation.
It is now reasonable for me that the significant variation of mooring force (with the lowest close to zero) can happen for this extreme condition especially considering such a high current speed.