Problem

In modal analysis, a model may show local modes when certain members, such as vertical ducts or risers, are defined as dummy, while the same model shows global modes when those members are defined as structural elements. This behavior occur even when the retained degrees of freedom (DOFs), geometry, and nodal configuration remain unchanged, and the overall mass appears to be similar. 

Answer

This behavior is expected and is related to how SACS handles dummy structures during analysis. 

When a portion of the structure is defined as dummy, SACS includes its weight (mass) and any applied loads, but removes its stiffness from the model. The solver transfers equivalent forces and moments to the KEEP joints, and the dummy structure is eliminated from the stiffness formulation before the dynamic analysis is performed. As a result, the structure contributes mass but does not contribute stiffness to the system.

Although Dynpac extracts modes using the retained DOFs, the analysis is based on reduced stiffness and mass matrices derived from the full structural system. Since dummy members do not contribute stiffness, the system can become locally less constrained in those regions. This reduction in stiffness coupling may lead to localized modal behavior, particularly in the lower modes.

In this particular case, the effect was amplified by the local distribution of mass and stiffness around the affected region. Even though the dummy structure stiffness may be relatively small compared to the overall model stiffness, local modes are governed primarily by local stiffness and mass distribution rather than the global stiffness magnitude. This effect can become more pronounced when connected to elements such as wishbone members.

When the same members are modeled as structural elements, both stiffness and mass are included. This preserves structural connectivity and load paths, resulting in global mode shapes. It is also important to note that the removal of stiffness for dummy structures occurs before the retained DOFs are applied, so retained DOFs alone do not fully govern the system response.

In practice, the difference is typically most noticeable in the first few modes, while higher modes tend to show good agreement. This indicates that the effect is localized rather than a general inconsistency.

Resolution

The local modes can be resolved by running Seastate analysis first to generate the OCI file, and then use the generated OCI file as input for Dynpac. With this approach, the expected global modes were obtained and the localized modes were no longer observed. 

This behavior is consistent with SACS Dynpac methodology and does not indicate a program defect.