100 m. would be the absolute minimum in benign and/or semi protected waters, like the Java Sea, where anything over 3 m. seas and 40 kts. sustained wind is rear.
Even so it would be difficult to justify it, if any possibility of thruster of positioning fault is taken into the safety planning.
I know that an anchored drillship worked in very shallow water (40 m.) by digging a "cellar " in the seabed and keep a very high work tension in the mooring system.
PS> This was the SEDCO 601, which could drill with surface stack, or sit on bottom in even shallower water.