Learning Forum

[Mike Pettit]

Hello,

This may depend on how big your structures are. Ansys Aqwa is particularly suited to structures with relatively large displacement, i.e. smaller Keulegan-Carpenter numbers, where the drag forces are less significant compared to the inertia (and radiation/diffraction) forces.

In Aqwa we usually describe a structure using panel elements, then we calculate the pressures on those panels, and hence the resultant forces on the structure. The solution method assumes that the surrounding fluid is inviscid, so there are no drag forces automatically included in the calculation. However, Aqwa can solve the Morison equation for 'slender' components, where the size/diameter D of the component is small compared to the wavelength L of the incoming waves (D < L/7). So, if your geometry is made up of one or more large components (e.g. a ship) or a mixture of large and small components (e.g. a semi-submersible rig with slender braces) then Aqwa is the best choice. The large components should be modelled as surface bodies, while the small components should be modelled as line bodies (beams).

In this case, for your structural analysis you would run the hydrodynamic calculations with Aqwa, then link the time domain Hydrodynamic Response system to a Static Structural system. In this way you can transfer the (global) surface pressures and structure motions calculated by Aqwa into a (local) structural analysis. However, this is only possible for Static Structural calculations - there is no capability to map to Transient Structural (yet) - so the inertial loads would not be included.

On the other hand, if your structure only consists of slender components, you can do the analysis directly in Mechanical. Load the 'Offshore' Add-on into a Static Structural, Transient Structural, Modal and Harmonic Response system, and this allows you to define an Ocean Environment, including ocean current and regular or irregular waves. The Morison equation is solved for the beams or pipes in your structural model to provide the hydrodynamic loading to the structure.

Regarding computational cost, the radiation-diffraction analysis performed by Aqwa for a large displacement structure may take a few minutes to a few hours, depending on how refined you make the panel mesh. Time domain hydrodynamic calculations similarly may take minutes or hours depending on the simulation duration, number of cables, and analysis settings (i.e. whether we calculate pressures in a linear or nonlinear way). Structural calculation times in Mechanical are not significantly affected by including Offshore inputs.

In terms of solving for many sets of MetOcean data, this is easy with Aqwa, as we can set various ocean properties (wave height, current direction etc) as input parameters, and then Workbench can run the parameter study for us. It is not so easy with the Offshore Add-on, as we can only run one case at a time.

I hope this helps! Please let me know if you have any further questions.

Mike