# Coupling between the TTMD and platform motion

Dear All

I have a question about the coupling between the TTMD(TMD placed in tower base) and platform motion(like platform pitch) of the offshore wind turbine. I am wondering if the platform pitch will directly influence the displacements of TTMD like the TTMD is placed directly on the platform or the motion of TTMD is influenced by platform pitch through something?

Besides, I am wondering how the tower for-aft displacements will influence the motion of TTMD as I thought the TTMD is placed on the tower base where tower displacements is very very small, which is not like the position of NTMD(TMD placed in nacelle) where tower displacements are very big.

If the TTMD is not directly placed on the platform, I am wondering how to model the coupling between the TTMD and platform and tower displacements as I am constructing a mathematical model of offshore wind turbine. I have modeled the tower and nacelle assembly to an inverted pendulum. The inverted pendulum is hinged at the platform with constant spring and damper. I want to add the TTMD to this model but not sure how does the TTMD coupled with platform and tower.

Best

XING WEI

Dear Xing,

Good question; I see that the FAST documentation is not clear on this.

In FAST v8, the at-rest position of the tower tuned-mass-damper (TTMD) is rigidly connected to the flexible tower, which itself is cantilevered to the moveable platform. That is, the TTMD will be located TMD_P_Z meters along the tower above the tower base and rigidly offset from the tower centerline by TMD_P_X and TMD_P_Y in the local transverse directions. So, the at-rest position of the TTMD will follow the displacement of the platform and deflection of the tower, both of which will induce motion of the TTMD when its degrees-of-freedom are enabled. If TMD_P_Z <= 0 (i.e. at or below the tower base), i.e. where the tower deflection is zero, then the TTMD will only be influenced by the platform motion.

I hope that helps.

Best regards,

Dear Jason

Thank you for your reply! I tried to set TMD_P_Z < 0 to see the effects while there is an error which means that I have to set this value equal or larger than 0. I am wondering if I set TMD_P_Z = 0, then how will the TTMD influence the tower as the at-rest position of TTMD is higher than the tower base by 0 meters, which means that the arm of force exerted on the tower by spring and damper is 0 meters. Does this variable “TMD_P_Z” correspond to the arm of force exerted on the tower by spring and damper or what is the method we should use to calculate the moment exerted on the tower by TTMD?

Besides, I am wondering if the platform motions have direct influence on TTMD motions? For example, I am wondering if the platform pitch will cause the TTMD to incline downwards so that TTMD will move under the effect of its gravity (disable other platform DOFs). If not, does the platform influence the TTMD through tower or through other way? The reason why I ask this is that I am adding the TMD dynamics into a inverted pendulum model. I am struggling on how to add in the moment exerted on tower by TTMD and how to influence the motion of TTMD by platform and tower motions.

Best

XING WEI

Dear Xing,

I’m not aware of a limit that TMD_P_Z cannot be less than zero. What exactly is the error you are receiving?

As I said in my prior post, the at-rest position of the tower is located a distance of TMD_P_Z along the tower; all the loads (forces, moments) from the TMD will be applied to the tower at this point. The forces applied at this elevation will also induce moments about the tower base (based on distance TMD_P_Z, when the tower is undeflected), but it is better to think of the loads being applied locally at distance TMD_P_Z along the tower, as this will impact the tower motion as well.

Yes, the platform motions will have a direct influence on the TTMD motions, as you describe.

I hope that clarifies things.

Best regards,

Dear Jason

Thank you very much for your reply. I double checked and found that TMD_P_Z cannot be less than 0. The error I encountered when I set TMD_P_Z<0 is shown in the following figure.

From your last post, My understanding is that all the loads from TTMD will be applied to the at-rest position of the tower, which is located a distance of TMD_P_Z along the tower. The definition of TMD_P_Z is the distance from tower base to at-rest position so I am wondering if I set TMD_P_Z = 0, then all the force from TTMD won’t induce moments on the tower as the forces are all applied on the tower base? If TMD_P_Z > 0, then the arm of force from TTMD will be different when the tower is deflected or not. But once we set TMD_P_Z = 0, I think TTMD should not have any impact on tower motions as the forces from TTMD are all applied on tower base.

Please correct me if any of my statement is wrong. Thank you in advance.

Best

XING WEI

Dear Xing,

Yes, your understanding is correct.

Regarding the error for a TTMD when TMD_P_Z < 0, I now understand that this error is related to the mesh-mapping in the FAST glue code rather than within the ServoDyn/TMD module itself. This makes sense to me given the limitations of the mesh-mapping algorithm implemented in the FAST glue code. The result is that for the TTMD, TMD_P_Z must lie between 0 (for the tower base) and TowerHt-TowerBsHt (for the tower top) (inclusive).

Best regards,