Dear @Yajun.Ren,
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.
Best regards,