How many US Flagged MODUs exist?

Does anyone have a list of the number of US Flagged MODU’s. I’m specifically looking for Dynamically Positioned MODUs.

I’m also wondering why each is US flagged. For example, the GSF Explorer is U.S. Flagged because it’s owned by the US governemtn (right?) but why did Helix energy flag the Q4000 US?