This is a drilling structure which is used in relatively shallow water, usually 80 feet or less. It is towed to its location where it is submerged until it sits on the bottom. This submerging serves as its mooring systems, although anchors may also be used.
In the background, just above the derrick, is a production platform. Notice there is no derrick on it. Approximately five miles separate the two platforms in this picture.
Photo by Ron Edwards