It does look like its possible to work offline https://knowledge.autodesk.com/supp...ow-to-work-in-offline-mode-in-Fusion-360.html
but does look like it has some restrictions http://help.autodesk.com/view/fusion360/ENU/?guid=GUID-FCB6E122-1552-4434-AC4B-7B227AB31BAC
There was a change in licensing for fusion360 recently, which is displayed as (personal - not for commercial use). Since then I don't remember having to sign in when starting fusion. Ive just tried it with out an internet connection and its starts ok for me, but only the items I've designed since the change are available to load.
https://knowledge.autodesk.com/supp...-or-educational-licensing-for-Fusion-360.html
Click the link for 'register for a personal license'