Drilling vessels bottom-supported

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. 

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. 

Bottom-supported vessels consist of drilling platforms, jackup rigs, and drilling barges. A drilling platform is not employed until reserves warranting field development is indicated. Platforms are currently being set in water depths up to 1,000 ft, and also serve as permanent production facilities. 

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. 

---