From my lecture notes|
so far, the maximum water depth that a fixed structure has been installed is the Bullwinkle jacket structure in a depth of 412m.
For intermediate water depths of 200m to 400m, floating structure are competitive alternatives to fixed structures.
For deep water (>400m), floating structures are the only viable option for developing a field.
The growing importance of floating structures is linked to the present industry's focus on oilfields in the deepwater (>500m) and ultra-deepwater(1000m to 3000m) range.