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luwang00
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Apr 10, 2026
luwang00
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Apr 10, 2026
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I believe this PR is ready to be merged.
r-tests using SoilDyn are expected to return different results.
Feature or improvement description
In PR #1526, the static improvement method (SIM) was removed from the elastic displacement output in SubDyn's Y3 mesh. The rationale was that SIM-corrected displacements were not consistent with the elastic velocities and accelerations, which do not account for SIM contributions from truncated higher modes. This change was introduced to avoid issues with MoorDyn in floating systems.
However, this change introduced an inconsistency for fixed-bottom systems: the full elastic displacement at the reaction node in SubDyn (Guyan + CB + SIM) differs from the displacement passed to SoilDyn via the Y3 mesh (Guyan + CB only, no SIM).
To overcome this limitation, the Y3 mesh for fixed-bottom systems has been modified. The proposed code modifies only the displacements of the reaction node(s) to include the full elastic displacements (Guyan+CB+SIM). All other nodes in the Y3 mesh retain their existing Guyan + CB displacements, preserving consistency between displacements and their time derivatives (velocities and accelerations). This should also ensure correctness for MoorDyn connections to non-reaction nodes in fixed-bottom systems. The only unaddressed edge case is a MoorDyn connection directly to a SubDyn reaction node, which is not an expected configuration.
Related issue, if one exists
Extensive documentation and testing can be found here: #3293
Impacted areas of the software
SubDyn Y3 mesh output for fixed-bottom systems. I believe this mesh is currently used by SoilDyn and MoorDyn.