Dear @Jason.Jonkman,

I ran a decay test for the tower in fore aft and side side. I set inflow, aerodyn and inflow to zero. In hydrodyn, still water is considered. The results TTDspFA and TTDspSS are weird.

TTDspFA is not decaying with time. (it seems to me logic because no aerodynamic damping) whereas TTDspSS is decaying with time. Here after images of TTDspFA and TTDspSS:

What do you think ?

Best Regards,

Riad

Dear @Riad.Elhamoud,

Are you running an OpenFAST model provided by NREL, e.g., one available from the r-test? Does your model have structural damping or hydrodynamic damping (radiation damping, viscous drag)?

Best regards,

Dear @Jason.Jonkman,

Sorry for the late reply. My model isn;t provided by NREL. It is the OpenFast model for the IEA 15 MW mounted on the spar floater named Windcrete. This spar floater was designed under the project corewind.

In fact, i found it weird that in a decay test the tower response in fore does not decay with time.

Moreover, i wanna ask you why do i obtain a moment around x at the tower base in the case of aligned wind and waves. In fact, i ran a simulation using steady wind of 11 m/s at hub height aligned with x axis and a wave aligned also with the wind. I should obtain at the tower base a moment around y axis only. I was surprised that i have a moment around x-axis and also around z-axis (torsion).

Could you please explain to me where do these moments come from ?

Furthermore, i know that OpenFast is a MDOF software. The equations of motion are obtained using Kane method. However, i was a little surprised when OpenFast gives moments (for example at tower base). I think (i am not a 100 % sure) that these moments aree computed from deflections (Euler bernoulii beam delfections). My question is : Is there any reference which explains how do OpenFast compute moments (blade roots or tower base) ?

Best Regards,

Riad

Dear @Riad.Elhamoud,

The wind load is likely resulting in an aerodynamic torque that will cause side-side deflection of the tower and sway/roll of the rotor. Rotor gyroscopics also imply that platform-pitch motion will induce yaw motion.

Rather than computing the tower loads as displacement*stiffness, the ElastoDyn module of OpenFAST computes tower loads using a load summation method. That is, the tower-base bending moment is calculated in ElastoDyn by integrating along the tower the ( radial vector ) x ( applied aerodynamic forces + gravitation forces - inertia forces ), plus the tower-top effects. All of these forces, moments, and distances are vectors and are oriented with the tower as the tower deflects.

Best regards,

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Dear @Jason.Jonkman

Thank u for your fast reply.

i wanna ask : if the wind speed is too high (cut out regime), does this aerodynamic torque persist or it will be suppressed due to the fact that the blades are feathered ?

Because the aerodynamic torque you are talking about (moment around x), i am thinking that it comes from the normal aerodynamic loads ( we have two aerodynamic loads: thrust load and normal load. The thrust is the main cause of the tower bending in fore aft and based on what i understand, i think that the normal component is responsible for the moment around x or side side moment ).

What do you think regarding what i’ve said ?

Best Regards

Riad

Dear @Riad.Elhamoud,

I would say that when the rotor is parked or idling with the blades feathered to 90 degrees and the rotor is kept aligned with the wind (little skew), that both the aerodynamic thrust and aerodynamic torque are greatly reduced relative to the thrust and torque obtained via normal turbine operating conditions.

Best regards,

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