Semi submersibles are used by large drilling companies to assess the viability of a location in deeper waters since unlike a jack-up rig; it is able to float on the water. Very often, this vessel is used to conduct exploratory and extracting functions and if the area turns out to have a substantial amount of oil, a drilling platform will be built.
A semi submersible is also called a floater. It consists of a barge or ship with 6 to 8 cylindrical legs beneath the hull. The drilling set up is located on top of the vessel. At the base of the legs, pontoons and ballast tanks exist to help position and move the barge as the need arises. When the vessel has to be positioned, the ballast tanks and pontoons are filled with water to lower the vessel to the appropriate depth. Then using dynamic positioning, anchors are lowered to keep the vessel in place for drilling operation. Both the anchors and the ballast tanks play a vital role in keeping the vessel stable, which is essential for smooth and effective drilling operations.
When the floater has to be moved, the water is emptied from the ballast tanks and pontoons, the anchors are raised again using dynamic positioning and the vessel is towed to its new location. Some of the more sophisticated semi submersibles are even able to move by themselves with the help of thrusters. When the new location is reached, once again the process begins to effectively position the rig.