Analysis of Riserless Drilling and Well-Control Hydraulics

Summary 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 diameter 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 balancing internal and external subsea well pressures. However, these concepts were not implemented at that time because water depths were shallow, and technology was not available. In the Gulf of Mexico, exploration attempts have been made in areas which have more than 7,000 ft water depth. A conventional large diameter riser requires a vessel with huge weight and space capacities, large mud volumes to circulate through a riser, and numerous casing points because of the relatively low separation between the formation pore pressure and fracture pressure, especially in the Gulf of Mexico. These problems 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 currently has many unsolved problems such as system configuration and well control. This paper presents basic concepts of riserless drilling and a brief review of problems associated with conventional marine riser drilling for deep water applications. The paper also presents hydraulics and well control considerations for riserless drilling with comparison to conventional riser drilling.