A semi-submersible is a form of oil platform, typically used for drilling, consisting of a deck supported by pillars connected to pontoons. When ballasted, the pontoons are entirely underwater, minimizing the waterplane area, thus providing a platform stable from ocean motions. The relatively large separation still provides a large waterplane inertia, which maintains the rigs stability. It is differentiated from fully-submersible rigs in that it does not rest on the bottom of the sea. The disconnect from the seabed and modest motions allow semi-submersibles to operate in deeper waters, however in particularly deep waters the mooring arrangement can become excessively heavy and unwieldly due to the length of lines. For particularly deep waters (i.e., >10,000 feet), tension-leg or spar platforms are often better suited. Semi-submersibles are held in place by either active or passive mooring systems.
Passive mooring systems rely on a system of wire, chain and anchors spread out about the rig to maintain position. As the chain rises or falls with the rig motion, it provides a restorative force which opposes the rig. In deep waters, the length required to reach the bottom can be considerable. Shallow waters can present a problem as well, as only the chain provides resistance (the anchor could pull out if subjected to a vertical loading) - in order to have a sufficient amount of restoring force, chain becomes heavy and long. Anchor handling vessels are required to help lift, move, and position the large anchors.
Semsisubmerisble oil rigs offer a number of advantages over other oil rigs;