Analysis of riserless drilling and well-control hydraulics

Riserless drilling is an unconventional technique using a relatively small diameter pipe as a mud return line from the sea floor instead of a large di ameter marine riser. The schemes were developed in the late 1960s to reduce wear on blowout presenters and to make drill pipe re-entry easier by balan cing internal and external subsea well pressures. However, these concepts w ere not implemented at that time because water depths were shallow, and tec hnology was not available. In the Gulf of Mexico, exploration attempts have been made in areas which have mon than 7,000 ft water depth. A conventiona l large diameter riser requires a vessel with huge weight and space capacit ies, large mud volumes to circulate through a riser, and numerous casing po ints because of the relatively low separation between the formation pore pr essure and fracture pressure, especially in the Gulf of Mexico. These probl ems may be reduced significantly by applying riserless drilling. Therefore, riserless drilling is one of the attractive alternatives for economically exploring oil fields in deep water. The concept of riserless drilling curre ntly has many unsolved problems such as system configuration and well contr ol. This paper presents basic concepts of riserless drilling and a brief re view of problems associated with conventional marine riser drilling for dee p water applications. The paper also presents hydraulics and well control c onsiderations for riserless drilling with comparison to conventional riser drilling.