# Nodal displacements OpenFAST

Dear @Jason.Jonkman,

In OpenFAST, is there any way to obtain nodal displacements?

I am trying to simulate a scenario where an unmoored platform would rotate about the centre of floatation, given close to static conditions. For this, I need to know the displacement of two points on the platform between two static conditions (before and after imposing a small rotation).

I have tried using the nodal velocity components instead (M1N1STVxi, M1N1STVyi,M1N1STVzi), and integrate them to get the displacements, however this does not give the expected result. Could you clarify in what coordinate system these velocity components are defined, please? If I define the locations of the nodes in the HydroDyn input file:

---------------------- MEMBER OUTPUT LIST --------------------------------------
2 NMOutputs - Number of member outputs (-) [must be < 10]
MemberID NOutLoc NodeLocs [NOutLoc < 10; node locations are normalized distance from the start of the member, and must be >=0 and <= 1] [unused if NMOutputs=0]
(-) (-) (-)
1 1 1.0
2 1 1.0

are these updated with the current position of the platform, or do they remain at the position fixed in the global coordinate system?

I would be grateful for clarification.

Kind regards,
Kasia

Dear @Kasia.Patryniak,

The HydroDyn outputs you refer to (`M1N1STVxi`, `M1N1STVyi`,`M1N1STVzi`) are expressed in the global inertial frame coordinate system, as indicated by the `i` in the name.

Best regards,

Dear @Jason.Jonkman,

Thank you very much for your reply, I appreciate it.

Is there any way to get the nodal displacements of the chosen platform nodes?

Kind regards,

The displacements at the hydrodynamic nodes of the strip theory solution are not currently outputs available from HydroDyn.

If you are modeling the structure of the floating platform rigidly in ElastoDyn, you can output the 6 rigid body motions from which you can compute the displacement of any point in the platform. If you are modeling the structure of the floating platform flexibly in SubDyn, you can output the displacement of the structural analysis nodes from SubDyn.

Best regards,

Dear @Jason.Jonkman,

Thank you. Are the 6 rigid body motions outputs (“PtfmSurge” - “PtfmYaw”) defined in the global inertial frame coordinate system as well? Where is this system centered?

Kind regards,

Kasia

Dear @Kasia.Patryniak,

That is correct. The platform motions output from ElastoDyn express the motion of the ElastoDyn platform reference point in the global inertial frame coordinate system.

Best regards,