Abstract: Semisubmersible marine structure for operation in offshore waters, comprising a work deck which is supported by a buoyant substructure. The latter includes a separably connected anchor unit which can be controllably lowered to the floor of the offshore site and thereafter weighted, to regulate the position of the floatable structure. Tensioning lines extending between the anchor and the structure draw the latter downward below its normal floating disposition. Similarly, outboard anchor lines are actuated to locate the structure laterally with respect to its position over a drill site.