Platform surge in case of wave only

1

Dear @Jason.Jonkman ,

Hope you are doing well.
I am running a simulation on OPenFast in the case of periodic regular waves (WaveMod=1) only, without rotational velocity and in the case of a still air. I also disable aerodyn. The height of the wave is set to 2 m, the period peak is set to 10 seconds. I expect that the platform surge will be positive.
However, it is negative !!! :exploding_head:

image
image1040×560 13.4 KB

The wave profile is given in the following figure:

image
image751×452 13.1 KB

Could you give me your opinion ?

Best Regards,

Riad

2

Dear @Riad.Elhamoud,

Just a couple comments:

  • What is the surge motion without aerodynamics and in still water? It could be that the mean surge motion is negative due to coupling with platform pitch, which is likely negative due to the overhanging weight of the rotor-nacelle assembly.
  • Wave loads will only induce a mean load in the surge direction if second-order terms are enabled.

Best regards,

1 Like
3

Dear @Jason.Jonkman,

  • The simulation is done in still air and not still water. There is hydrdynamic forces but no aerodynamic ones.

Best Regards,

Riad

4

Dear @Riad.Elhamoud,

I understand that. My first question was asking you to run a case with still water to check if the overhanging weight of the RNA is the cause for the mean negative surge.

Best regards,

1 Like
5

Dear @Jason.Jonkman ,

I did a simulation where the initial conditions are all zero and without wind and wave.
You were right, the overhang lets the platform to offset in surge (negative surge). Hence, when an incident wave comes, the total surge is the sum of the surge due to overhang and the surge due to the incident wave (principle of superposition).
In my case, it appears that the offset of the platform due to overhang is greater than the offset due to the incident wave. That’s explain why the total surge of the platform is negative. correct ?

Moreover, the overhang is due to the offset of the blades, hub, and nacelle center of mass from the yaw axis right ? If i am saying was correct, so the aforementioned components create moments around the pitch and roll axis. But my question why the platform will offset while the tower can bend to retain these moments ?

The following figure shows the platform surge due to the overhang:

image
image1910×975 76.1 KB

Thank u in advance

Best Regards,

Riad

6

Dear @Riad.Elhamoud,

Yes, I agree with your conclusion.

The overhanging weight of the RNA causing a pitching moment that gets resisted by a combination of tower bending and platform pitch, and platform pitch is often coupled with platform surge, so the overhanging weight of the RNA can also induce platform surge.

Best regards,

1 Like
7

Dear @Jason.Jonkman ,

Thank you for your reply !

I have also a question: In the ElastoDyn summary file, in the subtitle Rotor properties, the position of each blade’s center of mass is given.
Could you please, tell me from where this distance is measured ? Is it measured from the hub CM when the blade is projected in the rotor plane e.g. (this distance takes into account the precone angle of the blade) ?

Thank you for your response,

Best Regards

Riad

8

Dear @Riad.Elhamoud,

In the ElastoDyn summary file (ED.sum), the individual blade center of mass, 1st mass moment, and 2nd mass moment are output relative to the blade root, not the rotor center. The distances are measured along the preconed blade-pitch axis.

Best regards,

1 Like