SSDT: Requires Visual Studio? Requires connection to SQL Server? (SQL Server 2016) I need to make some SSIS workflow packages. I see I can do this in SSDT.

I have not used SSDT before.

It appears that SSDT installs into Visual Studio. OK, I can do that. (I have VS 2015 installed.)

But my workstation has no visibility to the SQL Server. The SQL Server is firewalled, etc. 

Will I be able to author in a disconnected way in SSDT and then package a file that I can transfer to SQL Server?

If not, what is the workflow to author in the SSDT SSIS Designer and get the package to an isolated SQL Server?
Yes you can work offline but it will be difficult to develop and test without the input/output connections files/tables your package will use. 

One option is to install SQL Server on your pc, create a database and copy the source and target tables and some test data to your pc. Then you can develop and test locally.

Deployment will be difficult unless you have ssdt on the server. Otherwise you will need access to deploy to the server from another machine.