Dear MoorDyn Development Team,
I am currently using MoorDyn v2 coupled with WEC-Sim to simulate a multi-body floating system consisting of two hydrodynamic bodies connected by a catenary line. Both Body 1 and Body 2 are fully six-degree-of-freedom floating bodies, and the two ends of the line are attached to the corresponding fairlead points on the two bodies. The simulation is performed through the standard WEC-Sim–MoorDyn coupling interface.
During post-processing, I extracted the MoorDyn body-point and line-node outputs, including:
WECSimOutputs.moorDyn(2).Lines.BODY2PX
WECSimOutputs.moorDyn(2).Line2.Node4px
I initially assumed that these body-point and node-position outputs were expressed in the global inertial, or earth-fixed, coordinate system. Under this assumption, the MoorDyn position of the point attached to Body 2 should be directly comparable with the corresponding WEC-Sim Body 2 position.
However, I found that:
WECSimOutputs.moorDyn(2).Lines.BODY2PX
does not directly coincide with:
WECSimOutputs.bodies(2).position(:,1)
Instead, the following relationship is observed:
WECSimOutputs.bodies(1).position(:,1) ...
+ WECSimOutputs.moorDyn(2).Lines.BODY2PX ...
≈ WECSimOutputs.bodies(2).position(:,1)
This indicates that BODY2PX is effectively reported as a relative coordinate referenced to Body 1, the MoorDyn base point, or another internal coupling reference, rather than as the absolute position of the Body 2 attachment point in the global earth-fixed frame. The line-node outputs show similar behavior. For example, the coordinates represented by WECSimOutputs.moorDyn(2).Line2.Node4px appear to describe the node position relative to the same base or coupling reference. Therefore, when these node coordinates are plotted directly, the resulting catenary geometry is not represented at its actual position in the global earth-fixed coordinate system. In particular, the reconstructed line endpoint does not appear to move synchronously with Body 2 unless the motion of Body 1 or the MoorDyn base reference is added manually.
Could you please clarify the exact coordinate definitions of the following outputs in the WEC-Sim–MoorDyn coupled model?
BODYPX / BODYPY / BODYPZ
NodeX / NodeY / NodeZ
Are these quantities intentionally output relative to the MoorDyn base body or coupling reference point? If so, which position and orientation should be used to transform them into the global earth-fixed coordinate system?
More importantly, is there an existing MoorDyn output option that can directly provide the absolute global coordinates of all body attachment points and line nodes? If these global coordinates are already available internally in MoorDyn, could you please advise which output channels or settings should be used? Otherwise, is the intended procedure to reconstruct the global node positions during post-processing by applying the translation and rotation of the MoorDyn base body?
Clarification on the required coordinate transformation would be very helpful. This issue currently affects the visualization of the catenary line, the verification of the line endpoint motion, and the interpretation of the coupling between MoorDyn and Body 2. Any clarification on how to obtain or reconstruct the line-node and attachment-point coordinates in the global earth-fixed frame would be greatly appreciated.
Thank you very much for your time and continued work on MoorDyn.
Dear MoorDyn Development Team,
I am currently using MoorDyn v2 coupled with WEC-Sim to simulate a multi-body floating system consisting of two hydrodynamic bodies connected by a catenary line. Both Body 1 and Body 2 are fully six-degree-of-freedom floating bodies, and the two ends of the line are attached to the corresponding fairlead points on the two bodies. The simulation is performed through the standard WEC-Sim–MoorDyn coupling interface.
During post-processing, I extracted the MoorDyn body-point and line-node outputs, including:
I initially assumed that these body-point and node-position outputs were expressed in the global inertial, or earth-fixed, coordinate system. Under this assumption, the MoorDyn position of the point attached to Body 2 should be directly comparable with the corresponding WEC-Sim Body 2 position.
However, I found that:
does not directly coincide with:
Instead, the following relationship is observed:
This indicates that
BODY2PXis effectively reported as a relative coordinate referenced to Body 1, the MoorDyn base point, or another internal coupling reference, rather than as the absolute position of the Body 2 attachment point in the global earth-fixed frame. The line-node outputs show similar behavior. For example, the coordinates represented byWECSimOutputs.moorDyn(2).Line2.Node4pxappear to describe the node position relative to the same base or coupling reference. Therefore, when these node coordinates are plotted directly, the resulting catenary geometry is not represented at its actual position in the global earth-fixed coordinate system. In particular, the reconstructed line endpoint does not appear to move synchronously with Body 2 unless the motion of Body 1 or the MoorDyn base reference is added manually.Could you please clarify the exact coordinate definitions of the following outputs in the WEC-Sim–MoorDyn coupled model?
Are these quantities intentionally output relative to the MoorDyn base body or coupling reference point? If so, which position and orientation should be used to transform them into the global earth-fixed coordinate system?
More importantly, is there an existing MoorDyn output option that can directly provide the absolute global coordinates of all body attachment points and line nodes? If these global coordinates are already available internally in MoorDyn, could you please advise which output channels or settings should be used? Otherwise, is the intended procedure to reconstruct the global node positions during post-processing by applying the translation and rotation of the MoorDyn base body?
Clarification on the required coordinate transformation would be very helpful. This issue currently affects the visualization of the catenary line, the verification of the line endpoint motion, and the interpretation of the coupling between MoorDyn and Body 2. Any clarification on how to obtain or reconstruct the line-node and attachment-point coordinates in the global earth-fixed frame would be greatly appreciated.
Thank you very much for your time and continued work on MoorDyn.