Mike Pettit
Ansys Employee

Hello Greg,

To answer your questions:

When you want to run an Aqwa time domain calculation, the usual workflow is to connect Hydrodynamic Diffraction -> Hydrodynamic Response -> Hydrodynamic Response, where the first HR is a stability analysis and the second is the time response analysis. The stability analysis calculates an equilibrated position of the structure, which is used as the starting condition for the time domain calculation. Even if you have no moorings etc., the stability analysis will usually move the structure a small amount to account for any slight imbalance in its mass/displacement.

For your linked systems A -> B -> C -> D, the default behaviour is that the initial conditions for system D will be read from the final conditions in system C. In other words, the system D start time is the finish time of system C.

Generally it is better to run with a stability analysis first; for frequency domain Hydrodynamic Response it is essential. Define your environment in the stability analysis, then right-click > Propagate on the environment objects (waves, current, wind) to create linked copies in any downstream Hydrodynamic Response systems. This ensures that the environment is consistent between linked systems.

Having said this, the stability analysis only considers the (steady) drift effect of an irregular wave on the structure's equilibrium position, so you do not need to include the wave if you are not including drift in the time domain analysis. This is also why you can't add a regular wave to a stability analysis - it does not have any effect.

The Natural Modes results in Hydrodynamic Diffraction and Hydrodynamic Response stability analysis will probably be slightly different due to the small change in structure position in the latter. Add a Structure Position > Actual Response graph, with result inputs for each of the 6 degrees of freedom, to see how much the structure has moved in the stability analysis. The Connection Points won't have any influence on the Natural Modes results.

In AqwaGS, there should be a legend across the top of the merged plot which indicates the line colours/quantities:

Cheers, Mike