Drilling vessels floating

Offshore drilling vessels are classified as either bottom-supported or floating-type vessels. Water depth is generally the governing factor as to which type of vessel is employed. 

Drillships and semisubmersible rigs are the two types of floating drilling vessels. 

In offshore drilling, the equipment allows drilling from a floating vessel and the completion of oil wells on the ocean floor is accomplished by remote control from the surface. The drilling vessel uses an automatic pilot to keep it in position while drilling. 

The basic principles of rotary drilling defined for onshore operations are also applicable to offshore operations. The primary difference offshore is that a stable, self-contained platform must be provided for the drilling equipment. Communication with a well through possibly thousands of feet of water provides for mechanical as well as procedural differences, primarily in well control. Onshore technology can be applied to offshore operations in many instances on bottom-supported rigs, but the use of floating vessels has resulted in the development of new technology tailored to the offshore environment. 

While a bottom-supported vessel must divert when shallow gas is encountered, a floating vessel has the additional option of simply abandoning the well. This option has led to the use of riserless systems when drilling the surface hole. However, a dynamic kill provides the only means of controlling the well. A dynamic kill makes use of annular friction as well as a heavier mud to hold backpressure on the formation. If very short wellbores are involved, the dynamic kill rates are usually to large to be practical. A well being drilled with a riserless system is very likely to be lost if shallow gas is encountered. 

In deeper waters, exploration is conducted from floating rigs, including submersibles and drill ships. Drill ships with conventionally shaped hulls of seagoing vessels are not so stable as semisubmersibles (semis) in rough waters, but can to moved from location to location much faster-... 

The number of agencies, and the complexity of their relationships, makes their coordination challenging. (The situation onshore for refineries and chemical plants is much simpler.) For example, when a floating drilling rig is moving to its location, it is a marine vessel under the control of a captain who has to comply with the U.S. Coast Guard standards. However, once the rig is on station it is under the control of an Offshore Installation Manager (OIM) and under the jurisdiction of BSEE. 

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