The dry-docking (DD) operation took several hours.  The sequence involved moving the dry-dock away from the pier and slipping in a spare barge between the dry-dock and the pier.  They sunk the dry-dock so that the floor of the dock was about 6" deeper than the steel barge.  They then floated the steel barge onto the dry-dock and raised the DD until it was just touching the bottom of the steel barge at which point the steel barge was flooded to neutral buoyancy.  Next, they slowly sunk the entire thing until the concrete float was sitting about 6" deeper than it was expected to float.