In deepwater drilling, a conventional large-diameter riser requires drilling vessels with large weight and space capacities, large mud volume to circulate through a riser, and many casing points because of narrow gaps between pore and fracture pressures. A large number of casing points also require a larger wellhead and a larger marine riser. These problems are inter-related and intensify as the water depth increases. Although there are some successes to set a new drilling record on water depth, it is impractical to extrapolate current technology with a marine riser to 10,000 ft water depth.
Subsea mudlift drilling (SMD) is a term used to describe an unconventional technique using a relatively small diameter pipe as a mud-return line from the seafloor instead of a large-diameter marine riser. The scheme also balances internal and external pressures at the seafloor by reducing the internal pressure to make a dual pressure gradient possible. It has potential advantages of cost and time savings and rig upgrades for deepwater applications.
Kick detection and well control will not be hurdles for field applications of SMD. If the circulation rate is less than the maximum free-fall rate, there will be a time delay in kick detection. In this case, the circulation rate may need to be increased or a drill-string valve that provides a positive surface-pump pressure should be used. In case of transient U-tubing or fill up, other operations should be avoided for easy kick detection and well control, unless there is an accurate prediction and monitoring tool available.