Semi-Submersible Rig
A semi-submersible rig is a floating offshore drilling unit. As the name suggests, a semisubmersible rig is partially submerged in water. The rig floats on hollow steel cross-sections called "Pontoons." Some semi-submersible rigs have propulsion systems that allow them to navigate drilling sites under their own power, while others require a second vessel to tow them to the location. Most rigs use several anchors that help maintain the structure's orientation. Computers control the tension on each anchor chain to correct for drift. Since the rig doesn't float on the water surface, it provides a more stable drilling platform than a drillship. Semisubmersible rigs are used for drilling wells in 1,000 feet to 3,500 feet of water depth.