Although marine riser for drilling operations is the industry standard, this process is more and more expensive in deeper water. Fifty years of technology development and study applied to marine riser do not alleviate the direct and associated expenses which increasingly impact economic feasibility of drilling deepwater wells. In addition to marine riser costs, associated tools and running time add tremendous expenses to deepwater well construction. Large riser mud volumes in deeper water increase potential environmental impact in the event of equipment failure. Rig selection and construction may be driven by increased variable deck load and increased surface to seafloor volumes of drilling mud, which can exceed rig pit capacity. Mud treatment rates, blending and bulk transfer requirements increase costs during operations, as well as storage or disposal of larger mud volumes when drilling operations are suspended or intervals reach total depth.
Recent concepts introduced to reduce marine riser size or to eliminate marine riser for mud and cuttings return from the mudline up to the drilling rig hold promise for deepwater operations. However development costs may be as great as the expense of deepwater riser operations which they are intended to address.
A new concept has been developed allowing deepwater drilling applications and circulation using mud return line instead of marine riser. Combined drilling technologies enable drilling mud return conduits and associated tools. The conduit allows cuttings to return to the surface while drilling without requiring a marine riser or seafloor booster pumps. The concept requires a housing for the subsea rotating control device (RCD) as an annular barrier and modifying subsea BOPE. Removal of marine riser, mud volumes and associated equipment using this riserless drilling system will extend MODU load capacity and deck space requirements. The impact of this riserless drilling method with potential upgrades promises to substantially extend the existing floating rig fleet generation's operation depth.
The method and concept are described in this paper. The primary enabling tools consist of an "E-duct Return" (EdR) subsea unit, with associated flexible composite pipe providing the drilling conduit and cuttings return designed to permit bidirecitonal flow; a closed circuit BOP and control housing; subsea BOP with integrated guiding choke and kill lines; and "intelligent" drillpipe, BHA and bit with downhole motor.
In additon to removing the riser from deepwater well construction, the method and tools described in this paper enable dual gradient drilling and potentially can be used in deepwater top hole riserless drilling with returned cuttings.
Figure 1 - Riser consumes large amounts of deck space. (Available in full paper)
Various methods of riserless and dual gradient drilling have been developed over the past decade. All have the common goal of removing the hydrostatic head associated with the mud column in the riser. The methods vary from using different types of subsea mud pumps to blending glass beads in the mud system as a means of lowering annular pressures to the well bore.