In the geometry editor (e.g. SpaceClaim) you need to right-click > Detach on the bottom surface of the Foundation structure (i.e. any part that is touching the seabed), so that it appears as a separate Surface Body in the geometry. Re-import the geometry into the Aqwa Workbench editor, then click on the new Surface Body you just created. In the Details panel, change the Surface Type from Program Controlled to Non-Diffracting. This should allow you to proceed with the analysis.